A closed level control system for vehicles of the above type is disclosed in U.S. Pat. No. 6,726,189. The level control system shown in this patent includes two or four controllable directional valves, a pressurized medium supply vessel and at least two pressurized medium chambers having respective controllable directional valves. With this level control system, a pressurized medium from the pressurized medium chambers can be transferred into the pressurized medium supply vessel or vice versa. Furthermore, air from the atmosphere can be transferred into the pressurized medium supply vessel. The compressed air is dried in an air dryer before entry thereof into the pressurized medium supply vessel. To regenerate the air dryer, pressurized medium from the pressurized medium supply vessel can be expanded via a throttle and, in comparison to air drying, be conducted in the opposite direction through the air dryer via a relief valve to the atmosphere. With this level control system, no abrupt different control speeds can be achieved as is desirable and required for specific application purposes.
U.S. Pat. No. 4,834,418 discloses a closed level control system for a vehicle and includes: a compressor, an air dryer, an intake valve, a discharge valve, two low-pressure pressurized medium supply vessels and two high-pressure pressurized medium supply vessels. The low-pressure and high-pressure pressurized medium supply vessels are connected via respective directional valves to the compressor input or the compressor output. Likewise, pressurized medium supply chambers are connected via controllable directional valves to the compressor input or the compressor output. The pressurized medium chambers can be connected to the low-pressure pressurized medium supply vessels or the high-pressure pressurized medium supply vessels via controllable directional valves so that pressurized medium can be exchanged in accordance with the pressure drop. With the level control system, pressurized medium can be transferred from the low-pressure pressurized medium supply vessels or the pressurized medium chambers by the compressor into the high-pressure pressurized medium supply vessels. A transfer of pressurized medium from the high-pressure pressurized medium supply vessel into the low-pressure pressurized medium supply vessel or the pressurized medium chambers with the compressor is not possible notwithstanding the complex configuration with many controllable directional valves.